Asphyxia continues to be a major cause of fetal death and newborn depression. There is relatively little information on the effects of asphyxia under unanesthetized conditions on the fetal cardiovascular system, the compensatory adaptations of the fetus to non-lethel asphyxia, and the physiologic mechanisms of these adaptations. We intend to examine the effects of deliberately imposed asphyxia on fetal oxygen consumption, regional distribution of blood flow, fetal heart rate and blood pressure and some aspects of carbohydrate metabolism. Asphyxia will be produced by controlled decreases in uterine blood flow with arterial constrictors for relatively prolonged periods (30 min). In addition, responses of heart rate and fetal oxygenation to brief periods of cessation of uterine arterial blood flow (20 sec) will be examined under conditions of partial fetal asphyxia. The role of each branch of the autonomic nervous system in producing cardiorespiratory responses will be examined by selective pharmacologic blockade with appropriate agents under conditions of asphyxia. It is hoped to gain insights into fetal compensatory mechanisms and to relate these in particular to fetal heart rate changes. This may lead to more rational approaches to the interpretation of fetal heart rate as seen in clinical obstetrics, and possibly to new treatments for the fetus threatened with asphyxia.